(a) Field of the Invention
This invention relates to an improved preparation process for organic isocyanates from their corresponding organic primary amines (hereinafter simply called "amines") and phosgene. More particularly, it relates to a continuous preparation process for organic isocyanates in two stages, both under pressure, from their corresponding amines and phosgene.
(b) Description of the Prior Art
Organic isocyanates, especially, aromatic polyisocyanates such as tolylenediisocyanate (hereinafter called "TDI") and diphenylmethane diisocyanate (hereinafter called "MDI") have been produced recently in large amounts by phosgenating their corresponding primary amines in an inert organic solvent such as o-dichlorobenzene. Great profitability may be derived from even a small amount of economical improvement, if such an improvement is incorporated in the preparation of a product which is produced in large amounts as mentioned above. Accordingly, such an improvement has an extremely important significance from an industrial viewpoint.
A number of processes have been proposed to obtain such isocyanates through reactions between their corresponding amines or salts thereof and phosgene.
Many of such prior art processes are however accompanied by such drawbacks that they cannot provide any high yield over 90% or, even if such a high yield is available, they require maintaining the concentrations of their reactants at a very low level or they take an extremely long period of reaction time. Thus, they have been found unsuitable for application on an industrial scale.
Many of such prior art processes make use of a reaction method in which, in the case of TDI for instance, liquid phosgene or an inert solvent solution of phosgene is reacted at a low temperature below 30'-40.degree. C. with an inert solvent solution of its corresponding amine so as to convert a part of amino groups into carbamyl chloride or amine hydrochloride and the resultant reaction mixture is then heated to 100.degree.-180.degree. C. to convert both carbamyl chloride and unreacted amine hydrochloride, the latter having been byproduced in the course of the reaction, into the isocyanate by feeding phosgene into the reaction mixture.
According to the above reaction method, the reaction may be carried out in various reaction vessels, including tank-shaped reaction vessels, tower-shaped reaction vessels and recirculating lines. It has also been proposed to conduct phosgenation by using each amine and phosgene in a special molar ratio or to carry out phosgenation in the presence of a special catalyst or under high pressures. ##STR1##
In Equation (1), the reaction between the amine and phosgene proceeds very fast even at low temperatures. In the course of the reaction, an amine hydrochloride is by-produced. The formation of carbamyl chloride through the reaction between the thus-formed amine chloride and phosgene--Equation (2)--is relatively low and can barely take place at low temperatures. Accordingly, it is commonly carried out to heat, while feeding phosgene, the reaction mixture to 100.degree.-180.degree. C. so as to convert unreacted amine hydrochloride into the isocyanate and, at the same time, to decompose the carbamyl chloride into the isocyanate in accordance with Equation (3).
Here, carbamyl chloride starts to decompose rather readily by heating the same and is decomposed almost completely at a temperature of about 120.degree. C. or higher. However, the reaction velocity between unreacted amine hydrochloride and phosgene is relatively slow. If the reaction temperature is raised to accelerate the latter reaction, the isocyanate resulting from the decomposition of carbamyl chloride tends to react with the amine hydrochloride, leading to the formation of an undesirable urea compound. The urea compound in turn reacts with phosgene or undergoes polycondensation with the isocyanate, resulting in the occurrence of varied complex side reactions and the formation of tar-like substances as by-products. Thus, the use of a high reaction temperature has been considered as a major cause for a lowered yield.
A variety of improved processes have also been proposed to conduct the phosgenation of amine hydrochlorides at as low a temperature as possible and to obtain isocyanates with a good yield. For example, Japanese Patent Publication No. 14664/1964 (U.S. Pat. No. 3,184,494) discloses a method in which a reaction of Equation (1) is carried out at a temperature near room temperature in a first step, the reaction product of the first step is then completely converted into carbamyl chloride in a second step in accordance with Equation (2) by effecting the conversion at 60.degree.-80.degree. C. for 4 to 8 hours, and the carbamyl chloride is decomposed in a third step in accordance with Equation (3) into isocyanate so as to obtain the isocyanate with a good yield. This method is however not satisfactory from an industrial viewpoint for producing TDI or MDI in large amounts because it requires a very long reaction time and the concentrations of reactants cannot be raised too much and it thus requires a reaction vessel of a large capacity.
It may be considered to make the first step by continuously conducting the reaction at 60.degree.-80.degree. C. with a view toward obviating the above-mentioned drawbacks. Unless the reactants are well mixed at the initial reaction stage, by-products will be increased and carbamyl chloride will deposit on the walls of the reaction vessel as disclosed in Japanese Patent Laid-open No. 56635/1978 (U.S. Pat. No. 4,096,165). Moreover, an additional reaction vessel is required to achieve sufficient residence time so that the reactants can be completely converted into carbamyl chloride.
As prior art publications disclosing a process enabling mass production in a reaction vessel of a relatively small capacity may be mentioned for example Japanese Patent Publication Nos. 10774/1960, 17381/1965 and 6126/1976 (U.S. Pat. Nos. 2,822,373; 3,234,253; and 3,544,611 respectively). According to Japanese Patent Publication No. 10774/1960, an isocyanate-containing solution is recirculated along a closed loop at a carbamyl chloride-decomposing temperature or higher and an amine is charged into the thus-recirculating isocyanate-containing solution so as to cause the amine to react with the isocyanate immediately. Then, by-produced hydrogen chloride and excess phosgene are removed under low pressures at a certain point, thereby recovering a part of the isocyanate-containing solution. Therefore, the amine and phosgene are brought into contact in a short period of time in an eddy of a high Reynolds' number and the amine can be converted immediately into its corresponding isocyanate. Reasonable results have been achieved by this method, since organic isocyanates can be continuously produced with a relatively high yield when the concentrations of their corresponding amines are kept relatively low, i.e., in the range of 5-10%. However, more amine is reacted undesirably with the isocyanate, besides the reaction between the amine and phosgene, as the concentration of the amine in the reaction mixture becomes higher, resulting in the formation of byproducts, non-volatile tar-like substances and leading to a lowered yield. Thus, the above method is accompanied by a drawback in that it does not permit the use of amines in a high concentration.
According to Japanese Patent Publication No. 17381/1965, an amine is catalytically reacted with excess phosgene to form a corresponding isocyanate and an intermediate. Before the prestage reaction (100.degree.-110.degree. C.) has been completed, in other words, after a lapse of several seconds to 30 minutes, the reaction mixture is transferred into another reaction system which is maintained at 150.degree.-170.degree. C., where the amine is brought into contact with a mixture of phosgene and hydrochloric acid while maintaining the concentration of hydrochloric acid in the mixture higher than that in a reaction mixture formed through the complete reaction between the phosgene and amine introduced into the first reaction system. The above publication discloses specifically that it is of particular importance for improving the yield of each isocyanate to control the residence time of the reaction mixture in the first reaction system and to adjust the ratio of phosgene to hydrochloric acid within a predetermined range in the second reaction system. The above method has not yet been found to be a fully satisfactory one because it is too complicated to apply it on an industrial scale and it does not permit the use of reactants in high concentrations.
According to Japanese Patent Publication No. 6126/1976, phosgene and an amine are reacted at a temperature of 40.degree.-120.degree. C. and a gauge pressure of 10-50 Kg/cm.sup.2 and the carbamyl chloride in the thus-formed reaction mixture is then heated at a temperature of 120.degree.-180.degree. C. and a gauge pressure of 15 Kg/cm.sup.2 to continuously prepare an organic isocyanate. In one embodiment of the process disclosed therein, the reaction between phosgene and an amine is carried out while recirculating a reaction mixture through a tubular line which includes at least one gas-liquid separater. The subsequent heating step of the resultant carbamyl chloride is conducted under pressure in a distillation column. In the distillation column, the decomposition of carbamyl chloride into an isocyanate and the evaporation of hydrochloric acid and phosgene take place. A dilute isocyanate mixture, which contains phosgene and drawn out of the distillation column from a bottom part thereof, is subjected to distillation under atmospheric pressure or a slightly raised pressure, thereby isolating an isocyanate-containing solution. As has been known conventionally, the solubility of phosgene in the reaction mixture can be increased to a substantial extent if the reaction is carried out under high pressures such as 10-50 atoms. Such an increased solubility of phosgene serves to accelerate the velocity of the preparation reaction of carbamyl chloride and allows an isocyanate with a good yield to be obtained even if the reactants are present in high concentrations. If the decomposition into carbamyl chloride in the second step is carried out at a high pressure, the condensation of unreacted phosgene can be carried out easily, thereby bringing about an effect that the problem of separating hydrochloric acid can be made easier. However, it is very dangerous from an industrial viewpoint to handle a reaction liquid containing excess phosgene under high pressures. Special consideration must be taken from the viewpoint of safety.
Carbamyl chloride has a low solubility to solvents. When slurry of such a high concentration is recirculated through a tubular line under high pressures, the degree of its corrosiveness to metallic materials increases considerably and commonly-employed stainless steel cannot be used, leading to a substantial increase in the cost of plant and equipment, for example, due to requirement for expensive materials.
Furthermore, the use of the reactants in high concentrations increases not only tar-like by-product substances but also increases the viscosity of the carbamyl chloride slurry to a considerable extent, resulting in the occurrence of tubular line cologging and making the recirculation of the reaction liquid difficult. On the other hand, where a high pressure is employed only in a single stage as in the method disclosed in Japanese Patent Publication No. 6126/1976, sufficiently long residence time is required to complete the reaction in the first sstep because, unless the conversion of the amine hydrochloride formed in the course of the reaction into carbamyl chloride is completed, the decomposition reaction of carbamyl chloride as a second step in a distillation column is impeded by the clogging of the column due to the remaining amine hydrochloride and the yield of the isocyanate is lowered.